copyright 2002 Leipper Management Group. All rights reserved
Please address comments or questions to webmaster@leipper.org
Last updated
Note: there are a lot of facts, assumptions, and calculations that are used to provide the information presented here. The intent is to provide 'back of the envelope' accuracy and not engineering or science level accuracy. There may also be cases of teaching license taken, such as the water and electricity analogy. However, errors or miscalculations may lead to misinformation or some analogies may mislead. If you see something you think should be corrected, please let us know - email scientist@SierraNevadaAirstreams.org
Energy and power are often confused which leads to interesting debates in areas like conservation of resources and pollution. For the RV enthusiast, the emotional aspects of this argument are overwhelmed by the practical aspects of having to carry sufficient energy in a form that can be easily and safely transported and converted to the power needed to operate a recreational vehicle and all of its accessories.
Energy is the capability to do something. We store it and buy and sell it. Energy has to be moved to where it is needed at an appropriate rate and time and, usually, converted into another form before we can realize its capability. The process of moving energy at an appropriate rate is called power. Power requires conveyances such as tubes, pipes, or wires to move the energy depending upon its form and how much needs to be moved how fast.
The basic problem with energy is that we need to be able to store enough of it in order to have it available when we need it. Sufficient storage is needed to meet our needs for the time we are away from resupply points. An RV enthusiast also needs energy in a convenient and compact form that is reasonably safe on the road and in camp.
The most common energy source for the RV is based on the storage of hydrogen that can be chemically combined as needed with oxygen from the air to produce water and heat energy. Emerging fuel cell technologies combine these chemicals to directly produce electricity but they have a ways to go before they are practical for RV use[1]. The sun is another common energy source but it is very diffuse and its use is like having a gas tank with a small feed pipe and a shutoff valve we can't control. For those who RV in submarines or aircraft carriers, a small nuclear plant makes a very nice way to store a lot of energy in a very compact and dense form whose conversion is easily controlled and produces very little environmental impact.
The means we use to store hydrogen is usually by attaching it to carbon in chemical compounds called hydrocarbons. These can store two or more hydrogen atoms on each carbon atom. The more complex the collections of carbon and hydrogen atoms (called molecules) get, the more hydrogen is packed into smaller volumes. The problem is that these molecules contain stuff other than hydrogen which complicates the energy conversion process. Fuel cells need clean hydrogen. Internal combustion engines don't care as much but all of the extra crud means the combustion will produce a lot of stuff besides water. The carbon burns to carbon oxides. Since air is used for oxygen but it has seventy percent nitrogen, combustion also produces nitrogen compounds. All of this extra crud is what creates the pollution problem people worry about. And don't forget that even water vapor is considered a green house gas which some worry about, too. Burning a gallon of gasoline will produce nearly eight gallons of water - just look at how rusty your tailpipe is!
Methane uses one carbon molecule so it is very light and has be very cold to become a liquid. Propane has three carbon atoms and butane four which makes them heavier and easier to liquefy. Octane gives the hint that gasoline has about eight carbon atoms in each molecule which makes it heavier and a liquid at normal room conditions. Liquid is perhaps the easiest form of material to use for commerce as it is easy to store, easy to measure, and easy to transfer from place to place. In order to burn, though, fuels need to vaporize and mix with air. So the hydrocarbons that are easy to liquefy, but barely so, are the fuels most common for RV use.
The theoretical measure of energy is the joule which is one full watt for one full second. Energy is also commonly measured as the BTU which equates the energy used to how much and how fast the energy can can heat water or the Kilowatt hour which means delivering a full thousand watts for a full hour. Methane delivered to houses use an energy unit called a therm, which is a bigger unit for billing convenience.
A BTU is the energy needed to raise a pound (pint) of water one degree Fahrenheit. A therm, often used for home gas supplies, is 100,000 BTU.
(small or gram) calorie - 4.184 joule - amount of energy to raise 1 gram of water 1 degree Celsius. There are 1000 of these in each Calorie used for food.
Power is the rate of flow of energy like speed is how fast the distance goes by. Its fundamental unit is the watt. Another common way to measure power is the horesepower (746 watts).
In electrical terms Power = Volts times Amps.
For comparison, figure an average household may use about 25 KwH per day. This costs about $2.50.
A typical type 27 battery that can deliver twenty amps over five hours at twelve volts and therefore stores about 1 kilowatt hour of energy. [ 20 amps * 5 hours = 100 amp hours or 0.1 AmpHours; 0.1 AmpH * 12 volts ~ 1 KwH]. Only about half of this capacity should be used as a general practice in order to assure longest battery life.
Propane energy density is 92000 BTU per gallon. At 3412 BTU per KwH this is 27 KwH per gallon or 189 KwH for a typical 30# or 7 gallon RV propane tank. Efficiencies in use go from perhaps 20% to near 90% depending upon application.
The sun provides about 1 Kilowatt of energy per square meter. Typical conversion efficiency for solar energy to electricity is about ten percent. A typical 20 foot long RV roof has about twelve square meters available for solar panels. The angle to the sun of solar panels on a flat roof would reduce their effectiveness another 50%. So an RV completely roofed with solar panels might get as much as 600 watts in full sun. That means about 6 KwH per day which would need 12 standard RV batteries to store it (50% battery capacity for best maintenance). A typical gigawatt commercial power plant would need many square miles of solar energy collectors and would not be able to store that energy, which is why commercial solar power is rather limited. (hydroelectric systems are most common for short term commercial energy storage as pumping water to a reservoir up the hill is an easy way to store a lot of energy)
250 horsepower, as in a typical RV engine, is about 180 Kilowatts. You use maybe a quarter of that to get down the road. A typical gas tank will last about four hours. So a tank is about 50 Kw for 4 hours or 200 KwH. Note that this appears about the same as a 7 gallon propane tank but the propane tank calculation was before use and this one is after use. This 200 KwH per tank of gas includes all of the losses involved in turning gasoline energy into miles traveled. At 1 KwH per typical vehicle battery, you can see why electrical cars are of limited practicability. 200 lead acid batteries at 50 lb each would weigh about 10,000 pounds!
A typical daily (24 hour) human diet of 2000 Calories is 4.184 joules per calorie times 1000 (gram or small) calories per (food) Calorie times 2000 Calories per day or 8 million joules over a 24 hour period. This is an average rate of about 100 watts (8 million joules divided by 86,400 seconds per day). i.e. You need to store about 2.5 KwH or food per person for each day you plan to be out of grocery store range. This human heat generation is why facilities designed for thousands of people have to worry a lot about cooling. At 100 watts each seat it can really add up!
[1] Fuel cells may someday be integrated into an RV black waste tank and garbage disposal. Methane from waste decomposition has been used to power vehicles (e.g. French farmers after WW I used trailers with manure based methane collectors to fuel their vehicles). Efforts to improve design and a bit of 'bug' breeding are aimed at generating hydrogen from biologic waste that can be used in a fuel cell to generate electricity. Meanwhile, Coleman has an RV hydrogen fuel cell about ready to market as soon as they can put together a distribution network for its hydrogen fuel system.